Abstract The innate immune system is responsible for the early non-self recognition events that lead to adaptive immunity. Although the mechanisms by which the innate immune system recognizes microbial non-self have been well defined, it is not known how the innate immune system senses allogeneic non-self. We have discovered that monocytes mount a specific response to allogeneic non-self independent of T, B, and NK cels. This response leads to persistent monocyte differentiation to mature dendritic cells (DC) after transplantation and is responsible for indirect alloantigen presentation. In this grant application we propose to define (1) the role of innate allorecognition by monocytes in acute and chronic rejection, and (2) the mechanisms by which monocytes sense allogeneic non-self. To accomplish these aims we will utilize transgenic and gene-knockout mice in which specific cell types and molecular pathways can be tracked or deleted. Genetic tools to identify mechanisms of innate allorecognition will also be employed. The proposed studies are innovative and significant because they represent a shift in our thinking about the innate immune response to transplanted organs and could yield novel targets for interrupting innate immune activation in a specific and safe manner.